1. Field of the Invention
This invention relates to a technique for converting a number of bits of digitally modulated channel bits and demodulating the same. More particularly, it relates to a technique for converting a number of bits of channel bits and demodulating the same in such a manner that signals from an optical disk may be commonly processed whether it is the DVD (Digital Video Disk) or the CD (Compact Disk).
2. Description of Related Art
As information distribution media employing the optical digital recording, there are CD and DVD. Different types of CDS include CD-ROM for read-only, CD-DA for audio, CD-WO for write-once and CD-R for recordable, all of which constitute the CD family by virtue of the uniformly defined recording/regenerating formats. Similarly, different types of DVDs constitute the DVD family.
Data bits of information to be recorded on CD or DVD are modulated into channel bits by its unique modulation scheme that complies with its unique RLL constraints, whereas a respective demodulating reference table is referred to for demodulating the original data bits during its playback. With regard to modulation of data bits, a notation "RLL (d, k)" represents that the number of successive `ZEROS` occurring between `ONES` in channel bits may be within the range of at least "d" (minimum run-length) and at most "k" (maximum run-length). Further, NRZ (NonReturn-to-Zero) or NRZI (NonReturn-to-Zero Inverted) recording method is employed to record data in such a way that a polarity of a bit `ONE` is inverted at its leading edge or center portion only to represent presence/absence of a pit on an optical disk, while such an inversion is not done for a bit `ZERO`. As a result, a modulation scheme that complies with its associated RLL constraints limits the minimum and maximum numbers of successive absences of pits on an optical disk. In so doing, during regeneration of information, it is possible to extract bit clock components for controlling a servo subsystem in a stable manner, and yet it is possible to obtain better regeneration characteristics that are free from wave interference due to a diffraction limit.
The EFM (Eight-to-Fourteen Modulation) modulation scheme has been employed for CD, whereas the 8/16 (alternatively called "EFMPlus") modulation scheme has been employed for DVD. One symbol of data bits is 8 bits for both of EFM and 8/16 modulation schemes, whereas one symbol of channel bits is 14 bits long for EFM modulation scheme and 16 bits long for 8/16 modulation scheme respectively. Note here that the term "symbol" means a word unit for error correction. Since both of CD and DVD employ different modulation schemes respectively, when one wishes to play back both of CD and DVD by means of a single playback apparatus, it has been required to provide signal processing circuitry (which is responsible for a sequence of operations, from detection of a sync bit pattern in a channel bit stream that enters into the demodulation subsystem up to output of regenerated data bits) for each of the modulation schemes respectively, in addition to provision of individual demodulating reference tables. FIG. 1 is a schematic block diagram showing a conventional optical disk playback apparatus that is capable of playing back both of CD and DVD.
As seen in FIG. 1, information on a medium 10 (CD or DVD) read by an optical head 11 is sent to a DVD data processor 16 and a CD-DA processor 17 via an RF preamplifier 12 as RF analog signals of a channel bit stream. However, data processor 16 or 17 operates to play back its associated medium alone. That is, data processor 16 including an 8/16 demodulator is only used during a DVD playback, whereas data processor 17 including an EFM demodulator is only used during a CD playback. After performing predetermined processing, its resultant data is sent to an interface 18 or a CD audio amplifier 19. Both of the 8/16 and EFM demodulators are provided with memories for storing demodulating reference tables, which are uniquely associated therewith respectively.
When channel bits of 16 bits long are treated as directly indicating an address of a ROM in the 8/16 demodulator for storing its demodulating reference table, the memory requires a space of 65,536 addresses. On the other hand, the demodulating reference table in the EFM demodulator requires a space of 16,384 addresses in its ROM since its associated channel bits are 14 bits long.